Task 3 Adding Shell Sort.cpp

[aniketsingh98571]

Is your feature request related to a problem? Please describe.
Shell sort is an algorithm that first sorts the elements far apart from each other and successively reduces the interval between the elements to be sorted. It is a generalized version of insertion sort. In shell sort, elements at a specific interval are sorted.

Describe the solution you'd like
Implementing using CPPSo

Describe alternatives you've considered
Sorting using Array

Additional context
Add any other context or screenshots about the feature request here.

[aniketsingh98571]

@helios1101 Please assign me this issue as a part of Hacktoberfest2020.